The need for cost effective improvement of mass storage capacity in disk drives has led to the development of redundant arrays of independent devices, such as disk drives, herein referred to as "disks". While disks are usually referred to in describing this invention, it is to be understood that such reference is only for convenience of disclosure and is intended to characterize other devices. The disks are electrically coupled in various arrays and are powered by a common electrical power supply. In these arrays, a typical disk subsystem comprises one or more disk controllers for accessing and controlling the function of individual disks, the functional relationships among the disks and the combinations of disk outputs, to present an appearance, in one RAID mode of operation, of an output from a single disk. Requests to read data from the disks or to write data to the disks are made by a host computer via the disk controllers which establish and maintain the communication link for each request. In this regard, array management software may execute in the disk subsystem (e.g., the disk controllers), or in the host computer, in orchestrating the RAID function.
The disks are electrically powered by an electrical power supply. When a disk is in operation in the system, the power demand for the disk spindle motor, the rotary actuator motor, magnetic head amplifiers, filter capacitors and other miscellaneous disk circuits is substantially constant and there are no significant power transients. The primary causes of power transients, however, at the time of connection of the disk to the power supply are the filter capacitors of the disk: drive. At the time of connection the capacitors are not charged. Since the capacitors are not charged they present a short circuit to the power supply at the time of connection. This is the primary cause of a significant power transient in the electrical power supply. Other types of devices may present similar power transient problems.
The disks are connected in parallel to the electrical power supply which also supplies power to all electrical components and circuits in the network, including those involved in critical management and logic functions. Power transients in the system, such as momentary power drops, may also disturb the disk speed or disrupt logic circuits sufficiently to disrupt critical RAID system functions, which is unacceptable. Maintenance of, or enlargement of, RAID system data capacity and reliability requires the connection of disks to the system from time-to-time while the system is in operation. Such a connection of a disk, or other device, to the RAID system, whether accomplished manually or automatically, is known as a "hot swap" or "hot plugging".
The suppression of power transients incident with the connection of a disk to the power supply in a RAID system has received continuing attention. One approach, involving the manual connection of a disk to the power supply of a RAID system, employs an electrical plug connector. The connector has extended length precharge pins. These "precharge" pins have a small resistor connected thereto. When connection is made, this resistor is coupled in series with the power supply. The function of the resistor is to limit current and to precharge capacitors on the device so that when the connection is made to the power bus with the shorter pins there is a smaller difference in voltage and, hence, a smaller transient.
There are a number of problems with this approach. First, the resistor often has only an intermittent use power rating, e.g., does not have a power rating sufficient to handle the full device current for a long period of time. Therefore, if the connector plug is not fully inserted into the connector receptacle, so that the short pins are quickly engaged, there is a risk of destroying the resistor. The use of a resistor rated for continuous duty to handle the current load, would mean very large and costly parts. The second problem is mostly a cost issue as connectors with multiple pin lengths are more expensive than connectors with only pins of one length. The third problem is that this method of transient suppression is very dependent upon device characteristics as well as timing involved with the insertion of the device, such as a disk.
There exists a need for a transient suppression system and method which are independent of device characteristics and in which timing is automatic.